The present invention relates to an inspection method and apparatus using an electron beam and, more particularly, to an inspection method and apparatus suitable for inspecting patterns on semiconductor wafers, photomasks and the like.
As semiconductor devices continue to shrink in size, greater sensitivity is required for detecting defects, foreign matter, and the like on semiconductor wafers and photomasks. In general, since a detection sensitivity of 1/2+L that of the wiring width of a pattern or less is required, the sensitivity of optical column defect inspection will reach its limit in the near future. In place of optical apparatus, an inspection apparatus using an electron beam has been developed and proposed in, e.g., Japanese Patent Laid-Open Nos. 5-258703, 7-249393.
Japanese Patent Laid-Open No. 7-249393 discloses a wafer pattern defect detection apparatus. This apparatus comprises an electron optical column 81 which has a rectangular electron emission cathode, and irradiates a surface 85 of a sample 82 with an electron beam, a secondary electron detection system 84 having a line sensor type secondary electron detector 86 for detecting secondary electrons 83 produced from the sample 82 irradiated with the electron beam, and a circuit 87 for processing the detection signal, as shown in FIG. 1. This defect detection apparatus is characterized by an ability to conduct high-speed pattern defect inspections on semiconductor wafers by setting the aspect ratio of a rectangular beam (which irradiates the sample surface), and executing parallel signal processing in a secondary electron detection system 84.
In relation to the resolving performance of a projection optical system for imaging a one- or two-dimensional image of secondary/reflected electrons produced from the sample, the electric field strength generated between the first electrode of the projection optical system and the sample can be increased and uniformity can be improved by placing the sample in the vicinity of the projection optical system. Hence, the projection optical means is placed so that its optical axis extends perpendicular to the sample surface.
However, conventionally, in order to realize such arrangement, the sample surface must be obliquely irradiated with a rectangular electron beam formed by a primary optical system due to the layout of the primary optical system and projection optical system in the vicinity of the sample surface.
Oblique incidence of the electron beam poses the following problems:
(1) When a pattern with a three-dimensional shape on the sample surface is obliquely irradiated with the irradiated beam, a region which is not irradiated with an electron beam is formed on the side opposite to the incident direction. For this reason, a portion opposite to the incident direction of the pattern appears as a shadow produced by secondary and reflected electrons. For this reason, it has been impossible to observe and inspect defects, foreign matter, and the like present on a pattern side wall, between adjacent patterns, and the like. PA1 (2) Upon irradiation with the electron beam, a negative voltage is applied to the sample. Because of this, when the sample surface is obliquely irradiated with the irradiation beam, the incident position of the electron beam onto the sample shifts from the original axis due to the influence of the electric field present between the sample and projection optical system. It is very hard to attain optical axis adjustment among the irradiated beam system, sample, and projection optical system due to the presence of the electric field. PA1 (3) The electric field is present between the sample and projection optical system, as mentioned above. When this electric field changes, the position on the sample surface irradiated with the electron beam moves, resulting optical axis adjustment errors. PA1 (4) When the electron beam is obliquely incident, electrons reflected by the sample have a distribution in the total opposite direction of the electron beam irradiated. For this reason, the transmittance of reflected electrons to the projection optical system whose axis is perpendicular to the sample surface had been reduced.
As described above, in the conventional method, since the sample surface is obliquely irradiated with an electron beam, it is impossible to inspect defects present on the pattern side wall, and is hard to adjust the optical axis, and so forth.